1. Field of the Invention
This invention relates to coated, abrasion resistant press plates used in making abrasion resistant decorative laminate, to the coating of press plates and to the making of laminate with these press plates. Grit, e.g., alumina particles, on the pressing surface of abrasion resistant decorative laminate can scratch press plates and reduce the visual quality of laminate thereafter made with the press plate. Press plates of this invention are particularly useful in making abrasion resistant high gloss decorative laminate.
2. Description of the Related Art
In the manufacture of decorative laminate, layers of resin impregnated paper are pressed against press plates under conditions of temperature and pressure to cure the resin and bond the layers together. A high gloss press plate imparts a high gloss surface to laminate. A textured surface imparts a textured surface to laminate. These press plates are extremely uniform, with even microscopic discontinuities being minimized. The quality of a high gloss polished press plate can be determined by viewing reflected images on its surface and scrutinizing the reflected images for optical discrepancies. Grit on the surface of laminate causes micro scratching of stainless steel press plates normally used in the manufacture of decorative laminate, thus destroying the micro finish of the press plate. Press plates can also be scratched by press plate handling equipment and by debris from pressing equipment or materials used in making laminate. (Laurence U.S. Pat. No. 5,244,375)
Melamine resin coated decorative laminate is pressed at temperatures of about 230-310xc2x0 (110xc2x0-155xc2x0 C.) and pressures of about 300-2000 psi (20-136 bar) and preferably about 750-1500 psi (51-102 bar). Heating to these temperatures and cooling to room temperature results in substantial expansion and contraction of the laminate and of the press plate. Expansion and contraction of the laminate and press plate will not be the same, resulting in the movement of grit on the pressing surface of laminate across the press plate.
It is disclosed in National Electrical Manufacturers Association (NEMA) Standards Publication No. LD 3, that gloss finish laminate has a gloss of 70-100+. High gloss textured finish laminate is disclosed as having a gloss of 21-40. Black glass with a gloss of 94+1, measured at an angle of 60 degrees, is disclosed as the NEMA Standard 3.2.2, for calibrating a gloss meter for 60 degree angle gloss measurements. 47526-PO24CP1-993 197 PATENT
Even discontinuities in high gloss press plates that can only be seen with a microscope can impart visible surface defects to a high gloss surface of laminate. Any scratching of high gloss press plates imparts visible surface defects to high gloss surfaces of laminate and reduce gloss level.
Grit on the decorative surface of laminate imparts abrasion resistance, a commercially desirable characteristic of laminate. Particles of alumina are commonly used as grit in making decorative laminate. The Vickers hardness of alumina is disclosed in xe2x80x9cTribology: Friction and wear of Engineering Materialsxe2x80x9d, I. M. Hutchings, CRC Press, 1992, to be 1800 to 2000. A useful range of particle sizes is about 10 to about 75 microns. Grit of about 25-60 microns is preferred. Optimum abrasion resistance is obtained in the particle size range of about 40 to 60 microns. (Lane et al. U.S. Pat. No. 3,798,111)
Alumina having a maximum particle size of 9 microns is disclosed as being effective for imparting a wear resistant surface to glossy decorative laminate. Wear resistance is defined as the resistance of a glossy laminate to loss of gloss when the surface of laminate is exposed to the abrasive effects of sliding objects. It is acknowledged that the resulting laminate does not meet NEMA LD 3.13 requirements to be considered as abrasion resistant. However, it is disclosed that glossy press plates are not scratched substantially if the grit particle size is maintained at less than 9 microns. (Lex et. al. U.S. Pat. No. 4,971,855).
The use of a 410 stainless steel press plate hardened by nitriding is disclosed for making high gloss decorative laminate. After pressing 100 sheets of high gloss laminate with 6 micron and 15 micron grit, the gloss of the pressed laminate remained good to very good. The nitrided press plate exposed to the 6 micron grit was rebuffed after 234 cycles and produced acceptable laminate quality for at least another 103 cycles. Nitrided press plates exposed to 30 micron grit offered limited durability. It is disclosed that the 410 stainless steel press plate used for nitrating had a Rockwell, xe2x80x9cCxe2x80x9d scale hardness of 38-45 and that the nitrided surface had a Rockwell, xe2x80x9cCxe2x80x9d scale hardness of 60-70. The equivalent Vickers hardness of 410 stainless steel is about 370-440, based on a conversion table published in xe2x80x9cMetals Handbook, Mechanical Testingxe2x80x9d, Vol 8, 9th ed., ASM, 1985. The equivalent Vickers hardness of nitrided 410 stainless steel is about 500-1000, based on a conversion table published in xe2x80x9cMetals Handbook, Mechanical Testingxe2x80x9d, Vol. 8, 9th ed., ASM, 1985. (Laurence U.S. Pat. No. 5,244,375)
Laminate with 35 micron average particle size alumina at its surface (PGA 822 overlay, available commercially from Mead Corporation) has been pressed with high gloss press plates coated with titanium nitride. After ten pressing, the titanium nitride coated press plates had about 15 scratches per square centimeter. A control 410 stainless steel press plate had about 500 scratches per square centimeter. The Vickers hardness of titanium nitride is disclosed in xe2x80x9cTribology: Friction and wear of Engineering Materialsxe2x80x9d, I. M. Hutchings, CRC Press, 1992, to be 1200 to 2000.
The control press plate and the press plate on which the titanium nitride was coated were cut from the same stainless steel pressing plate. The scratches was visible under a light microscope at 40X magnification. Titanium nitride was coated onto 410 stainless steel high gloss press plates in a magnetron sputter coating system. The use of a magnetron sputter coating system for applying a titanium nitride coating is disclosed in xe2x80x9cMulti-Cathode Unbalanced Magnetron Sputtering Systems,xe2x80x9d Sproul, Surface and coating Technology, 49 (1991). The use of a magnetron sputter coating system for cleaning the surface that is to be coated is disclosed in xe2x80x9cA New Sputter Cleaning System For Metallic Substrates,xe2x80x9d Schiller et al., Thin Sold Films, 33 (1976).
Additionally, the color of the laminate pressed with the titanium nitride coated press plate was different than the color of the laminate pressed with the control press plate. An ASTM D 2244 color difference in comparison to a standard of less than (xc2x10.5) xcex94E is considered as an acceptable color match to the standard. The ASTM D 2244 color difference between a standard and laminate pressed with the titanium nitride coated press plate was greater than (0.5) xcex94E. The titanium nitride coated press plate and laminate pressed therefrom bad a bronze appearance. The control press plate and the laminate pressed therefrom did not have a bronze appearance. Laminate pressed with the control press plate had an ASTM D 2244 color difference when compared with the standard of less than (0.5) xcex94E.
Iron-based cutting tools have been sputter coated with 2-6 microns of titanium diboride. The sputtering is carried out in an argon or krypton beam of ions accelerated to 1300-1800 volts as a broad-beam ion source. A titanium diboride target is arranged as a cathode. The tool is heated to about 200xc2x0 C.(392xc2x0 F.). Sputtering is done under a vacuum of about 4-6 milli-Torr. Titanium diboride has an extremely high Vickers micro-hardness value, typically about 3600, which is not only considerably higher than other borides but also substantially higher than other carbides or nitrides. Titanium diboride is also particularly noted for its high density, e.g., 88% of theoretical density, a low resistivity of 30 micro-ohms centimeters, a high strength of about 40,000 psi, and a coefficient of thermal expansion which is about 8.1xc3x9710xe2x88x926 at the temperature range of 20xc2x0-800xc2x0 C.(68-1472xc2x0 F.). (oskowitz et al., U.S. Pat. No. 4,820,392).
Control conditions for sputter coating are disclosed in xe2x80x9cInfluence of Apparatus Geometry and Deposition Conditions on the Structure and Topography of Thick Sputtered 20 Coatingsxe2x80x9d Thornton, Journal of Vacuum Science Technology, Volume 11, Number 4, (July/August 1974) and xe2x80x9cSputteringxe2x80x9d Thornton et al., Metal Handbook, Ninth Edition, American Society for Metals, Metals Park, Ohio 44073, Volume 5, pp 412-416, (1982).
There is a need for a hard coating on a press plates, continuous belt, and other pressing surfaces that imparts a color to laminate having an ASTM D 2244 color difference in comparison to a standard of less than (xc2x1.5) xcex94E. There is a need for a coating that can be applied to a pressing surface without changing the appearance of the finish on the pressing surface. There is a need for a pressing surface that is not scratched when used in pressing laminate coated with alumina particles of greater than 10 microns and preferably greater than 25 microns. There is a particular need for a pressing surface that is not scratched when used in pressing high gloss laminate with an ASTM 2457 60 degree angle gloss of greater than 70, when the surface of the laminate is coated with 25-60 micron alumina particles.
It has now been discovered that the color, gloss and surface appearance of laminate made with pressing surfaces coated with diborides selected from the group consisting of hafnium diboride, molybdenum diboride, tantalum diboride, titanium diboride, tungsten diboride, vanadium diboride, or zirconium diboride or mixtures thereof are substantially the same as the color and gloss of laminate made with the pressing surfaces before the coating is applied. The preferred diborides for coating laminate pressing surfaces are titanium diboride or zirconium diboride. The most preferred diboride for coating laminate pressing surfaces is titanium diboride. It is believed that titanium diboride is more commonly used commercially for coating surfaces than other members of the diborides of this invention because it can be sputter coated in a magnetron sputtering system at a higher deposition rate.
The diboride coating of this invention can be applied on laminate pressing surfaces to have a Vickers hardness of at least 2000 and preferably at least 2200, sufficient for pressing laminate with 25-60 micron or larger alumina particles at the pressing surface of the laminate without being scratched. A coating of about 3 microns has sufficient hardness to resist scratching by alumina particles on the pressing surface of laminate. The hardness of the coating can be controlled in a planar magnetron sputter coating system by those skilled in the use of these systems.
It has been discovered that the diboride coating of this invention can be coated on a pressing surface with sufficient bond strength for use in pressing high pressure laminate. A minimum bond strength of 1.6 and preferably 1.8 kilogram force (kgf) determined by diamond scratching bond testing is believed sufficient. Diboride coatings of greater than 6 microns can have lower bond strengths due to stresses produced during coating.
Bonding of the diboride coating of this invention to the pressing surface is enhanced by thoroughly cleaning the pressing surface before introducing the pressing surface into a magnetron sputter coating system. Bonding is further enhanced by etching the pressing surface with the magnetron sputter coating system prior to applying the titanium diboride coating. Cleaning, anodic etching, cathodic etching and etching with radio frequency (RF) can be accomplished by methods known to those skilled in the use of a magnetron sputter coating system. It has been discovered that a layer of titanium applied directly onto the pressing surface before applying the diboride coating of this invention further enhances the bonding of the diboride. Improving bonding by cleaning, etching and the use of an intermediate layer between the coating and substrate are known to those skilled in the art of using magnetron sputter coating systems.